One of the most enigmatic bodies in our solar system just got even more intriguing.

Cassini has flown by Titan more than 80 times since entering Saturn's orbit in 2004, and its observations have confirmed that, as moons go, Titan is a weird one. It's bigger than the planet Mercury. It's the only moon with a real atmosphere (an atmosphere denser than Earth's, in fact). It experiences Earthlike weather, such as rain and snow. It's home to familiar geological features like valleys, plains and deserts — and it's the only known object besides Earth with standing bodies of liquid on its surface.

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And yet these observations, while numerous, have all been skin deep. "In contrast," writes planetary scientist Luciano Iess, in today's issue of Science, "information on the moon's deep interior is scarce."

One does not simply drill into Titan, and there are no geologists on the moon's surface to measure its seismic waves. The absence of a detectable internally generated magnetic field means that everything we know about the interior of Titan has come from careful analysis of its orbit, rotation, gravity and topography. Fortunately for clever scientists everywhere, careful analysis can reveal incredible things.

To detect the tides of Titan, Iess and his colleagues had to get creative. Titan travels around Saturn in an elliptical orbit, experiencing the most gravitational pull as it approaches its closest point in orbit (pericenter), and the least at its farthest (epicenter). These variations give rise to tides, which squeeze at the moon's surface and cause it to flex. Tidal flexing leads to distortions in Titan's gravitational field that affect the speed at which Cassini approaches and recedes from the moon during flybys. It's this last bit — the effect of Titan's changing gravitational field on Cassini's velocity — that is ultimately measured by the spacecraft's onboard equipment.

The less dense the moon's interior, the more its surface flexes throughout its orbit, and the greater the distortions in the moon's gravitational field. Since Titan takes just 16 days to make a full trip around Saturn, Iess and his colleagues were able to use Cassini's velocity over the course of six different flybys to estimate the changes in Titan's shape throughout its orbit. The researchers calculated that if Titan were composed entirely of rock, the moon would experience bulges in its surface of up to one meter in high. Cassini's velocity measurements indicate the moon actually experiences bulges that are ten times that height.

When combined with data from previous research, including investigations into Titan's mysterious orbit, the researchers claim the most likely model of Titan's interior is one like the one pictured here, which depicts a global ocean located beneath an icy shell tens of kilometers thick. "Cassini's detection of large tides on Titan leads to the almost inescapable conclusion that there is a hidden ocean at depth," said Iess.

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The fact that the ocean's waters are located beneath a sheet of ice does not bode well for life; most experts contend that life is most likely to spring from places where water comes into contact with rock. Having said that, the models used by Iess and his colleagues have no way of telling whether the floor of Titan's subsurface ocean is made up of rock or ice, so Titan aliens are not entirely out of the question.

For now, however, most planetary scientists are interested in the ocean's role in maintaining the moon's diminishing atmosphere.

"The presence of a liquid water layer in Titan is important because we want to understand how methane is stored in Titan's interior and how it may outgas to the surface," said Cassini team member Jonathan Lunine.

"This is important because everything that is unique about Titan derives from the presence of abundant methane, yet the methane in the atmosphere is unstable, and will be destroyed on geologically short timescales."